Agreement is full, by the sound of it (4)
I believe the answer is:
pact
'agreement' is the definition.
(a pact is an agreement or deal)
'full by the sound of it' is the wordplay.
'full' becomes 'packed' (synonyms).
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'packed' sounds like 'PACT'.
'is' acts as a link.
(Other definitions for pact that I've seen before include "'Accord, treaty (4)'" , "deal" , "Treaty, formal agreement" , "Settlement" , "Agreement, covenant" .)